1. Field of the Invention
The present invention relates to an optical scanning apparatus used in an image forming apparatus such as a copying machine, a printer, a facsimile, or a multifunctional peripheral thereof.
2. Description of the Related Art
An electrophotographic image forming apparatus forms an image on a recording medium by making a scan on a photosensitive member by a light beam, developing an electrostatic latent image formed thereby with toner, and transferring and fixing a toner image thereof onto the recording medium. The electrophotographic image forming apparatus is equipped with an optical scanning apparatus to make a scan on the photosensitive member by a light beam. The optical scanning apparatus includes a light source to emit a light beam, a scanning unit (for example, a polygon mirror or rotating polygon mirror, hereinafter called a polygon mirror) to cause the light beam to scan on the photosensitive member by deflecting the light beam emitted from the light source, and an image forming optical system to cause the light beam scanned by the polygon mirror to form an image on the photosensitive member. For the image forming optical system, a scanning speed of the light beam scanning the photosensitive member is stabilized and spot shapes of the light beam formed on the photosensitive member are equalized by one optical lens or a plurality of combined optical lenses.
These optical lenses used for an image forming optical system are mainly formed of resin. Resin lenses have, compared with glass lenses, a disadvantage of being more likely to thermally expand when the temperature rises. However, it is necessary to give special optical characteristics to the optical lenses used for an image forming optical system and thus, optical lenses are formed of resin that is easier to form into special shapes. Moreover, resin lenses can be constructed more inexpensively in terms of cost compared with glass lenses. Due to the above advantages, resin lenses are mostly used as the optical lenses for an image forming optical system.
After the installation position of an optical lens is adjusted by a lens supporting member so that a desired position is scanned by a light beam when installed in an optical scanning apparatus, the optical lens is firmly fixed to the lens supporting member by a fixing member such as an adhesive or screw to hold the position inexpensively with precision (see Japanese Patent Application Laid-Open No. 3-182708).
However, if the optical lens is firmly fixed, issues described below may arise. Heat from a driving device driving the polygon mirror raises the temperature of an optical lens arranged near the polygon mirror, which in turn expands the optical lens thermally. If the optical lens is firmly fixed to the lens supporting member, a portion whose thermal expansion is limited by the fixing and another portion whose thermal expansion is not limited by the fixing arise, thereby distorting the shape of the optical lens and generating a possibility of changing the optical characteristics thereof. Moreover, if the optical lens is fixed to the lens supporting member by an adhesive and a force generated by thermal expansion of the optical lens becomes greater than a restriction part force, there is a possibility that the optical lens peels off from the lens supporting member to change the position of the optical lens.
Confronted with such issues, bringing the thermal expansion amount of the optical lens into line with that of the lens supporting member that fixes and supports the optical lens by bringing the linear expansion coefficient of the optical lens into line with that of the lens supporting member can be considered. Accordingly, even if the temperature of the optical lens and the lens supporting member rises, the optical lens is less likely to be distorted because the difference between both thermal expansion amounts becomes smaller. However, in view of the function of each member, it is very difficult to perfectly match the linear expansion coefficient of the optical lens and that of the lens supporting member. Moreover, the number of revolutions of a rotating polygon mirror is set higher for faster image formation in an image forming apparatus in recent years and thus, when compared with a conventional image forming apparatus, a calorific value from a driving device such as a motor that rotates the rotating polygon mirror during image formation is greater. Therefore, even if the optical lens and the lens supporting member are formed of materials having linear expansion coefficients close to each other, the difference between both thermal expansion amounts becomes greater when the temperature rises, leading to distortion of the optical lens.